left the 9-year-old city hall in ruins. archaeologist rebecca carver believes the newly discovered foundations belong to one of the few part of city hall that didn't collapse. >> what we with are see hoog is the top of the foundation or the eastern most part of the san francisco's old city hall. >> reporter: construction on old city hall started in 1872, but delays and bickering stretched the work across 25 years. >> by the time it finished, it had been built in stages and there's differences in construction technique. >> because of that architectural historian wants to understand better why that part of city hall survived. >> these foundations give us a clue how it was built differently. that's one of the thing we will determine if we compare what we learned about these foundations to foundations found elsewhere. >> old city hall was once a vast complex built on what had been a site of a cemetery. other parts were discovered during construction near the library and asian art museum. >> that is one thing that i like about h